While you wait, preheat the oven to 400°F (200°C) / 350°F (180°C) fan / Gas 6.
Bring a large saucepan of water to the boil. Add the baking soda (this will help give the bagels a shiny, chewy crust).
Plunge the bagels into the boiling water, 2 or 3 at a time depending on the size of your pan. Cook for 30 seconds on each side, until the bagels puff up and the shape sets. Allow the water to reach boiling point again between each batch of bagels you plunge.